Have you ever noticed those big metal pipes funneling water under roads? They’re culverts. And they are pretty unremarkable to the untrained eye — invisible from the road, their presence only betrayed by the occasional frost heave, detour, or drowned trees upstream.
To get a wholly different perspective, step into any river or creek and take a closer look where it intersects our transportation network.
Many culverts cause big problems for fish. When fish can’t get where they need to go it hits us in our pocket books, decreases our food security, and threatens millions of Americans’ recreational pursuits, traditions, and ways of living. Across the United States, migratory fish — like salmon, steelhead, and river herring — are particularly hard hit by barriers across streams and rivers.
Many culverts are also weak links in our nation’s infrastructure. Luckily, where there’s a problem there’s typically a solution.
Rivers and streams are powerful and ever-changing. They carry rocks and logs. They flood. They carve out canyons. And when they encounter a culvert not custom-built to their unique range of behaviors, they have no problem going over or through the road. Like this:
The problem with the majority of culverts underneath our residential streets, highways, and railways is that they’re not typically mindful of fish, floods, or the dynamism of rivers.
They’re almost always too small.
Each time a stream encounters a road there’s a good chance it’ll be pinched like an hourglass by a too-small culvert. It’s not uncommon for a 10 foot wide stream to be forced into a two foot diameter culvert. These choke points concentrate flows and make passage difficult or impossible for fish.
It’s kind of like being locked out of your house. Or finding the grocery store closed. For us, it’s an inconvenience. For fish, it’s potentially life-threatening. Their survival and success hinges on being able to move freely to find food and good places to spawn; or to seek shelter from predators, high temperatures, or extreme flows.
Some species and life stages are particularly vulnerable. For example, adult salmon are strong swimmers. Baby salmon are not. Barriers at roads created by too-small culverts can block them from getting to winter nursery habitats and reaching adulthood. One barrier culvert may not seem like a big deal, but each one chips away at the options available to fish. If fish have to put “all their eggs in one basket” so to speak, they’re at risk — and so are the fisheries we depend upon.
Natural variation in flows from year to year, especially during storms, can overwhelm too-small culverts, as seen here:
Too-small pipes also snag wood and debris moving downriver, much like a tub drain snags hair (gross). We all know what happens then: a clog. Roads with clogged culverts turn into dams until they’re breached, or the clog is removed. In the meantime, trees upstream drown.
They’re usually placed at or above the stream bed.
A stream’s bed can include boulders, cobble, gravel, sand, or even finer stuff. It moves. It creates roughness that breaks up the flow. Water moves much slower here than just a few inches higher in the stream’s main flow. Fish tend to pick the route of least resistance through these diverse particles: a large fish might rest behind a boulder, or hang out by a log, before making a short sprint through a riffle while a small fish moves from rock to rock along the bottom of the stream.
When a culvert pipe is set atop the stream’s bottom there’s only one option: a long stretch of smooth pipe at worst, corrugated pipe at best. The matter is further complicated when there’s a height difference between the outlet or inlet of a culvert and the stream-bed. These “perched” culverts are extremely common. They tend to become more perched over time as high velocities through the pipe carve out a plunge pool below.
Where roads and railways are concerned, what’s good for fish is good for us. Stream crossings designed to pass the weakest swimmers at all flows minimize the problems described above. Preventing barriers in the first place is the most cost-effective option long term.
A natural bottom.
What’s under our roads should ideally mimic what’s upstream and downstream. This helps ensure a seamless transition for fish passing underneath.
Wide enough to accommodate natural stream behavior and flows.
A stream’s width and depth varies day-to-day with how much water it’s carrying. So how wide is wide enough? To answer that, we must understand the stream’s range of flows. A stream gauge that tracks water level and documents flood events over time can help. But these are present on only a fraction of waterbodies — especially in Alaska. We can also measure the stream’s width where the water just starts to spill over onto the floodplain. This “bankfull” measurement is what we want. We need to construct a channel inside a culvert that matches the bankfull width of the stream. Ideally, banks can also be constructed inside the culvert to provide resting places for fish swimming upstream and dry ground for wildlife to cross through the culvert too. If we don’t want floods to overwhelm our roads, we need to give the rivers and streams they cross room to be their dynamic self — from their different flows down to their moving beds.
Perform well in floods.
Crossings that are sized to accommodate the expected range of flows naturally perform better when they encounter those flows. A case in point is the nearly 100 fish-friendly culverts installed in Alaska’s Mat-Su Valley; these culverts remained stable during a 100-year flood event in 2012. Their traditional round culvert counterparts required costly maintenance.
How do we get there?
To build quality transportation infrastructure that lasts we need to focus not only on water, but also fish — if they can move freely through road crossings to their preferred habitats we’ll know we’re on the right track. This means transportation planners and engineers engaging fish passage engineers, hydrologists, and fisheries biologists.
Be a part of the solution
- Share this message. We can all benefit from safer, more resilient transportation networks that also keep our waters and fish in good shape.
- Become a conservation partner. Our Fish Passage Program invests federal dollars and staff time into voluntary partnerships that improve fish passage where roads cross streams. Some projects are just too large and complex for any one person or entity to undertake alone. Get to know your local players.
- Attend a fish passage workshop. In Alaska, the U.S. Fish & Wildlife Service and the Alaska Department of Fish & Game offer multi-day workshops for planners, engineers, construction firms, and other practitioners interested in learning about fish-friendly crossings.
- Support efforts to document flows over time with stream gauges. We need more of this long term information everywhere, especially in Alaska.
- Join and support your local fish habitat partnership. Alaska has several (www.akfishhabitat.org) and fish passage is a shared priority.
- Find out if your local or state governments have fish-friendly road policies. Support efforts to prevent new barriers and remove existing ones. In Alaska, for example, several communities have adopted fish passage design standards (Anchorage in 2007; Kenai Peninsula in 2008; Mat-Su in 2013).
Katrina Liebich is based in Anchorage, Alaska and has served in her current position as Fisheries Outreach Coordinator in Alaska for the U.S. Fish & Wildlife Service since 2010. Question? Contact her: Katrina_Liebich@fws.gov or (907) 786–3637
In Alaska we are shared stewards of world renowned natural resources and our nation’s last true wild places. Our hope is that each generation has the opportunity to live with, live from, discover and enjoy the wildness of this awe-inspiring land and the people who love and depend on it.